The present invention relates to spectrophone or photo-acoustic systems and is more specifically concerned with a new and improved arrangement for eliminating noise from the spectrophonic measurement of gas absorption data.
Laser absorption spectrometers measure gas samples for the purpose of determining the absorption characteristics of the gas constituents. Measured absorption characteristics can be used to identify the presence of specific constituents in the gas. Laser absorption spectrometers can also be used to investigate the attenuation which is caused by certain gas constituents on radiation transmission through the gas.
One type of laser absorption spectrometer heretofore known comprises a sample chamber containing the gas to be analyzed. A laser beam is directed into the chamber through an appropriate window. Constituents of the gas will absorb energy of the laser beam in certain characteristic fashions, and subsequently deactivate giving rise to corresponding pressure changes within the chamber. These pressure changes can be detected to provide an indication of the absorption characteristics. The absorption characteristics in turn can be used to identify constituents in the sample and/or to ascertain the effect of the gases on the transmission of laser beam energy through the gas. In prior types of laser absorption spectrometers, pressure responsive sensors, or transducers, are positioned in the chamber to sense the pressure variations and to provide corresponding electrical signals indicative of these pressure variations.
In connection with the usage of such prior devices, the problem of background noise has been observed. It has been found that actual commercial laser absorption spectrometers have heretofore not possessed the required sensitivity for making certain types of measurements. Particularly in the field of measuring low absorption coefficients, background noise becomes a limiting factor especially at low total pressures.
The most sensitive photo-acoustic systems currently in use employ resonant chambers, or cavities, containing dynamic, or electret, microphones placed in such a way as to detect maximum pressure variations in the cavities. Since the detection limits of these systems are primarily a function of background noise level, differential type systems have heretofore been used in attempts to cancel the effect of background noise from the measured signal. These differential systems have typically consisted of two cavities in series, isolated from each other by an optical window. One of the cavities is filled with a non-absorbing, or reference, gas while the other is filled with the gas to be studied. Signals from the reference gas are taken as the background, or reference, signal and are subtracted from the signal obtained from the other chamber. A disadvantage of this arrangement is that it requires the use of two separate chambers (or sub-chambers) and in addition requires matching the response characteristics of the two chambers if proper background noise cancellation is to be realized. In addition, the background noise frequently limiting the detectivity of photo-acoustic systems is that arising from absorption of radiation in the optical windows of the system. Consequently, differential systems requiring a window between the two separate chambers (or sub-chambers) must completely cancel these relatively large window signals if they are to achieve their ultimate detection limit. An example of a differential system of this type is disclosed in U.S. Pat. No. 4,058,725, dated Nov. 15, 1977.
Other types of laser absorption spectrometers are disclosed in U.S. Pat. No. 3,793,525, dated Feb. 19, 1974; U.S. Pat. No. 3,727,050, dated Apr. 10, 1973; and U.S. Pat. No. 3,659,452, dated May 2, 1972.
The present invention is directed to a new and improved spectrophone containing an improved arrangement for elimination of background noise. One advantage of the invention is that only a single chamber is required thereby eliminating the complexity of having matched dual chambers of the differential type system described above.
A further advantage of the invention is that is does not require optical windows in the system for proper operation.
Briefly, pursuant to principles of the present invention, gas within a test chamber is subjected to an impingent laser beam from a laser which is operated in a manner as to impart a frequency to the chamber substantially at a resonant frequency of the chamber. This mode of operation sets up a resonant standing wave within the chamber possessing peak and nodal points. Sensors are located at the nodal and/or peak points, and signals from the sensors are utilized to develop the absorption signal; specifically, nodal point signals or peak signals 180 degrees out of phase are subtracted from one another. In the case of nodal point subtraction, the nodal point signals are predominantly the result of background acoustical noise and when substracted from the peak point signals serve to eliminate background noise from the peak point signals. In the case of subtraction of peak signals 180.degree. out of phase, the subtraction enhances the resonant component while canceling signals not possessing this specific phase relationship. In either case the result is a single chamber photo-acoustic detector with substantially improved noise rejection characteristics and a lowered detection threshold.
In the specific example of the invention disclosed herein, the resonant chamber, or cavity, is of an elongated cylindrical shape and the gas is excited at a longitudinal resonant frequency of the chamber. A set of microphones is located at the longitudinal midpoint of the chamber, and additional sets of microphones are located at or near the longitudinal ends of the chamber. When the gas is excited at the fundamental longitudinal mode frequency of the chamber, the center set of microphones measure the peak pressure variation while the sets of microphones at or near the ends of the chamber measure the pressure nodes. The measurements at the pressure nodes theoretically should be zero, and hence, any signal obtained other than zero is considered indicative of background noise. By averaging the signals from the end sets of microphones and subtracting the result from the average obtained at the center set of microphones, the background noise component is eliminated from the peak pressure measurement. The invention yields a substantial improvement in the signal-to-noise ratio of the system with attendant lower detection threshold capability which is particularly useful, as explained above, for measuring low absorption coefficients.
The invention also does not appreciatively impair the Q of the system and it avoids the adverse effect of the 1/f fall-off of the pressure signal characterizing other types of systems.
The foregoing features, advantages, and benefits of the present invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time for carrying out the invention.